A British botanist goes bananas after he discovers a serum that turns his cuddly chimpanzee subject into a ferocious gorilla-sized ape. To further his hideous experiment, the scientist mesmerizes the chimp and sends into London to kill all of his former enemies. One of those he has killed is the lover of the girl the doctor wants for himself. This doesn't set well with the botanist's assistant and current gal who gets even by giving Konga the giant chimp an enormous amount of the strange serum and turns him into a Godzilla-sized monster. Just before going on a deadly rampage, the super-sized ape grabs the bad doctor in one of his enormous hands. Fortunately, the British army and all of its weaponry are able to stop the chimp before he destroys the town.